1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to elevator systems which determine the absolute position of the elevator car in the hoistway.
2. Description of the Prior Art
U.S. Pat. No. 3,750,850, which is assigned to the same assignee as the present application, discloses a car position arrangement which uses incremental counting to determine car position. Car movement rotates the governor sheave, or a suitable pulse wheel, and a pickup arrangement detects spaced openings disposed in the sheave or wheel. The pickup arrangement generates pulses which are counted by a reversible car position counter. By resetting the counter to zero when the elevator car is located at the lowest floor, and then counting the pulses as the car travels upwardly, each floor of the building may be identified by a predetermined count in the counter. Thus, floor position memories may be programmed accordingly, and the floor selector may relate the count in the reversible car position counter with the counts of the floor position memories. While this arrangement has operated satisfactorily, it has certain disadvantages. For example, loss of power causes loss of the information in the reversible counter. Also, electrical noise in the circuitry may be counted as incremental car movement pulses. Stretch of the cable driving the governor sheave, and wear of the governor sheave and cable, cause positional errors which require reprogramming of the floor position memories. Finally, compression or settling of the building causes the floor addresses to change, also requiring reprogramming of the floor position memories.
Certain of the disadvantages of the incremental counting arrangement for determining car position can be overcome by determining the absolute position of the elevator car in the hoistway. In determining the absolute position of the elevator car, there are at least two criteria which must be met. Firstly, resolution should be 0.125 inch, or less, if the system is to land the elevator car without a separate landing device. If a separate landing device is used, the resolution must be 1.0 inch, or less, as this is the maximum allowable mismatch when transfer is made to the landing device. Secondly, the positional information must be updated every 10 ms or less, for system stability. Absolute car position could be determined by measuring the time for some form of electromagnetic radiation to travel between the car and a fixed point in the hatch. However, even using light, which has a velocity in air of 9.8.times.10.sup.8 feet per second, it would take 8.5.times.10.sup.-5 .mu.s to resolve 1.0 inch. This time differential to resolve a 1.0 inch increment far exceeds the speed capabilities of available solid state devices. Other techniques using electromagnetic radiation can be used, but they are complicated and costly.
Co-pending application Ser. No. 356,684, filed Mar. 10, 1982, entitled "Elevator System", by W. R. Caputo, which is assigned to the same assignee as the present application, discloses an absolute position elevator system which uses a coded tape. As further disclosed in this co-pending application, the absolute position of the elevator car can advantageously be used to develop a variable reference limit signal for the speed pattern clamp arrangement disclosed in U.S. Pat. No. 4,161,235, which is also assigned to the assignee of the present application. While the coded tape arrangement overcomes most of the disadvantages of the incremental counting arrangement for determining car position, it does not account for building compression, which, for example, may be as much as 12 inches for a building having a rise of 600 feet.